✅ Do we need to explicitly set foreign keys in EF Core?

Faker · 2025-02-25T20:36:11.260Z

Hello guys, just a quick question. Say I want to implement the following table which contains 2 foreign keys. Can EF Core automatically infer that the MemberId and BookId are foreign key (Assume we add navigation property in each table where needed)? ```C# namespace Week5.Models; public class Lending { public int LendingId { get; set; } public DateTime IssueDate { get; set; } public DateTime DueDate { get; set; } public DateTime ReturnDate { get; set; } // Foreign keys public int MemberId { get; set; } public int BookId { get; set; } // Navigation property - one lending can have one member; one lending can have one book; public Books? Book { get; set; } public Member? Member { get; set; } } ``` ```C# protected override void OnModelCreating(ModelBuilder modelBuilder) { // Book - Lending : One to Many modelBuilder.Entity () .HasMany(b => b.Lendings) .WithOne(l => l.Book) .HasForeignKey(l => l.BookId); // Member - Lending : One to Many modelBuilder.Entity () .HasMany(m => m.Lendings) .WithOne(l => l.Member) .HasForeignKey(l => l.MemberId); } ```

A

Angius•2/25/25, 8:36 PM

No need to

A

Angius•2/25/25, 8:36 PM

EF works by convention in this case

F

FakerOP•2/25/25, 8:36 PM

yeah I see, what does it look for it to identify the foreign key ?

F

FakerOP•2/25/25, 8:37 PM

things like {Book}Id ?

A

Angius•2/25/25, 8:37 PM

Property

[Name]Id

[Name]Id

will automatically be treated as a foreign key for property

[Name]

[Name]

A

Angius•2/25/25, 8:37 PM

Yep

F

FakerOP•2/25/25, 8:37 PM

Yep I see, thanks !

A

Angius•2/25/25, 8:37 PM

Same for the primary key

A

Angius•2/25/25, 8:37 PM

[ClassName]Id

[ClassName]Id

and

Id

Id

both work

F

FakerOP•2/25/25, 8:37 PM

yep noted

J

Jimmacle•2/25/25, 8:51 PM

also fwiw, when setting the relationship you can change either the id or the actual object, you don't have to change both

JJimmacle also fwiw, when setting the relationship you can change either the id or the act...

F

FakerOP•2/25/25, 9:35 PM

Yep, I guess its better to change the Id, no ?

J

Jimmacle•2/25/25, 9:36 PM

it depends on what you have at the time

J

Jimmacle•2/25/25, 9:36 PM

sometimes you have the object, sometimes you only have the id

J

Jimmacle•2/25/25, 9:36 PM

it's not worth a database roundtrip to pull the whole entity if you just need to change the id

J

Jimmacle•2/25/25, 9:37 PM

if you're creating new entities then you don't even have an id to set and you have to set the object

A

Angius•2/25/25, 9:37 PM

Could technically do weird shenanigans with manually attaching the entity you created from just the ID, but... don't

F

FakerOP•2/26/25, 7:12 AM

C#
// Option 1: Set Foreign Key directly
Lending lending = new Lending
{
    MemberId = 1,  // Set FK directly
    BookId = 10,
};

// Option 2: Assign the object instead
Lending lending = new Lending
{
    Member = existingMember,  // Set Navigation Property
    Book = existingBook,
};

C#
// Option 1: Set Foreign Key directly
Lending lending = new Lending
{
    MemberId = 1,  // Set FK directly
    BookId = 10,
};

// Option 2: Assign the object instead
Lending lending = new Lending
{
    Member = existingMember,  // Set Navigation Property
    Book = existingBook,
};

C#
// Option 1: Set Foreign Key directly
Lending lending = new Lending
{
    MemberId = 1,  // Set FK directly
    BookId = 10,
};

// Option 2: Assign the object instead
Lending lending = new Lending
{
    Member = existingMember,  // Set Navigation Property
    Book = existingBook,
};

C#
// Option 1: Set Foreign Key directly
Lending lending = new Lending
{
    MemberId = 1,  // Set FK directly
    BookId = 10,
};

// Option 2: Assign the object instead
Lending lending = new Lending
{
    Member = existingMember,  // Set Navigation Property
    Book = existingBook,
};

This is what you guys mean by either using the id or the object?

JJimmacle also fwiw, when setting the relationship you can change either the id or the act...

F

FakerOP•2/26/25, 10:20 AM

is this what you mean please

S

Sehra•2/26/25, 12:34 PM

did you try it?

F

FakerOP•2/26/25, 12:35 PM

For the option 1 yeah, I did it when creating a new lending object

F

FakerOP•2/26/25, 2:50 PM

This is what I did:

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet another book with another title" };
    context.Book.Add(book);
    await context.SaveChangesAsync();
    
    var books = context.Book.ToList();
    
    foreach (var b in books)
    {
        Console.WriteLine(b);
    }
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };
    context.Member.Add(member);
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = 2, MemberId = 1});
    await context.SaveChangesAsync();
}

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet another book with another title" };
    context.Book.Add(book);
    await context.SaveChangesAsync();
    
    var books = context.Book.ToList();
    
    foreach (var b in books)
    {
        Console.WriteLine(b);
    }
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };
    context.Member.Add(member);
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = 2, MemberId = 1});
    await context.SaveChangesAsync();
}

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet another book with another title" };
    context.Book.Add(book);
    await context.SaveChangesAsync();
    
    var books = context.Book.ToList();
    
    foreach (var b in books)
    {
        Console.WriteLine(b);
    }
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };
    context.Member.Add(member);
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = 2, MemberId = 1});
    await context.SaveChangesAsync();
}

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet another book with another title" };
    context.Book.Add(book);
    await context.SaveChangesAsync();
    
    var books = context.Book.ToList();
    
    foreach (var b in books)
    {
        Console.WriteLine(b);
    }
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };
    context.Member.Add(member);
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = 2, MemberId = 1});
    await context.SaveChangesAsync();
}

F

FakerOP•2/26/25, 2:51 PM

based on the book and member I added, I added a new record in Lending table

S

Sehra•2/26/25, 2:54 PM

so you can just change BookId and MemberId in the last query to refer to the objects you created earlier

J

Jimmacle•2/26/25, 3:04 PM

yeah 5 database roundtrips is not necessary, you can do it all in one

J

Jimmacle•2/26/25, 3:05 PM

also, you are inconstently using both SaveChanges and SaveChangesAsync

S

Sehra•2/26/25, 3:14 PM

if you added both a book and a lender at the same time, you wouldn't be able to use BookId since it doesn't have an id at that point

SSehra so you can just change BookId and MemberId in the last query to refer to the obj...

F

FakerOP•2/26/25, 4:21 PM

oh ok, but then I would do something like BookId = book.BookId ?

JJimmacle yeah 5 database roundtrips is not necessary, you can do it all in one

F

FakerOP•2/26/25, 4:21 PM

hmm what do you mean pls

JJimmacle also, you are inconstently using both SaveChanges and SaveChangesAsync

F

FakerOP•2/26/25, 4:21 PM

yeah was just experimenting with but there is nothing to do with Async at this stage I think, I should remove that

S

Sehra•2/26/25, 4:22 PM

just

Book = book

Book = book

in that case

SSehra just `Book = book` in that case

F

FakerOP•2/26/25, 4:23 PM

hmm when we create the new lending, like instead of using BookId and MemberId, we say Book = book and Member = member ?

JJimmacle yeah 5 database roundtrips is not necessary, you can do it all in one

F

FakerOP•2/26/25, 4:25 PM

oh I see what you mean, like the context.SaveChanges, thing ?

J

Jimmacle•2/26/25, 4:26 PM

3 pings in 4 minutes is crazy

F

FakerOP•2/26/25, 4:26 PM

ops, sorry :c

F

FakerOP•2/26/25, 4:29 PM

when writing

Book = book and Member = member

Book = book and Member = member

, behind the scenes, EF Core does the following:

Book = book.BookId and Member = member.MemberId

Book = book.BookId and Member = member.MemberId

?

J

Jimmacle•2/26/25, 4:37 PM

not necessarily

J

Jimmacle•2/26/25, 4:38 PM

if they're new entities then EF doesn't know their IDs yet assuming they're database generated

J

Jimmacle•2/26/25, 4:38 PM

it will produce an appropriate query for that

F

FakerOP•2/26/25, 4:43 PM

yeah I see, so the thing is, can we write something like that:

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet         another book with another title" };
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };

    context.Book.Add(book);
    context.Member.Add(member);

    // save both changes simultaneously, no need separate context.SaveChanges()         statements
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query); // query == book object

    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = book.BookId, MemberId = member.MemberId});
    context.SaveChanges();
}

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet         another book with another title" };
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };

    context.Book.Add(book);
    context.Member.Add(member);

    // save both changes simultaneously, no need separate context.SaveChanges()         statements
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query); // query == book object

    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = book.BookId, MemberId = member.MemberId});
    context.SaveChanges();
}

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet         another book with another title" };
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };

    context.Book.Add(book);
    context.Member.Add(member);

    // save both changes simultaneously, no need separate context.SaveChanges()         statements
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query); // query == book object

    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = book.BookId, MemberId = member.MemberId});
    context.SaveChanges();
}

C#
using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "Jane Doe", Isbn = 4588709, Title = "Yet         another book with another title" };
    
    // Add new member
    var member = new Member { FirstName = "Jane", LastName = "Doe" };

    context.Book.Add(book);
    context.Member.Add(member);

    // save both changes simultaneously, no need separate context.SaveChanges()         statements
    context.SaveChanges();
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query); // query == book object

    //Updating db
    query.Author = "Paul Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = book.BookId, MemberId = member.MemberId});
    context.SaveChanges();
}

Here, notice we have 2

context.SaveChanges()

context.SaveChanges()

... what I thought is, we must first create the book and member objects to have valid foreign keys else, we won't know what book.BookId and member.MemberId are.

J

Jimmacle•2/26/25, 4:43 PM

you also don't have to re-query to get the ID of a new entity you've saved, that should be automatically updated by EF after calling SaveChanges iirc

FFaker yeah I see, so the thing is, can we write something like that: ```C# using (var...

J

Jimmacle•2/26/25, 4:43 PM

you don't need to know what they are, EF can manage it

J

Jimmacle•2/26/25, 4:44 PM

it knows what relationships should be set based on the object graph and it will set FKs as part of the change saving process

F

FakerOP•2/26/25, 4:44 PM

oh ok, so I can just write a single context.SaveChanges() at the end provided that, I write something like that:

C#
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = book, MemberId = member});

C#
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, BookId = book, MemberId = member});

J

Jimmacle•2/26/25, 4:44 PM

it should work

F

FakerOP•2/26/25, 4:44 PM

yep noted, I will test and let you know, ty !!

S

Sehra•2/26/25, 4:45 PM

Book, not BookId

J

Jimmacle•2/26/25, 4:46 PM

i didn't say anything about that because the compiler will catch it :when:

F

FakerOP•2/26/25, 5:04 PM

Soo, so I write this:

C#
// CRUD Operations using EF Core

using Microsoft.EntityFrameworkCore;
using Week5.Context;
using Week5.Models;

using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "John Doe", Isbn = 4568709, Title = "Harry Potter" };
    
    // Add new member
    var member = new Member { FirstName = "LeBron", LastName = "James" };
    
    context.Book.Add(book);
    context.Member.Add(member);
    
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "John Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, Book = book, Member = member});
    context.SaveChanges();
}

C#
// CRUD Operations using EF Core

using Microsoft.EntityFrameworkCore;
using Week5.Context;
using Week5.Models;

using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "John Doe", Isbn = 4568709, Title = "Harry Potter" };
    
    // Add new member
    var member = new Member { FirstName = "LeBron", LastName = "James" };
    
    context.Book.Add(book);
    context.Member.Add(member);
    
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "John Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, Book = book, Member = member});
    context.SaveChanges();
}

C#
// CRUD Operations using EF Core

using Microsoft.EntityFrameworkCore;
using Week5.Context;
using Week5.Models;

using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "John Doe", Isbn = 4568709, Title = "Harry Potter" };
    
    // Add new member
    var member = new Member { FirstName = "LeBron", LastName = "James" };
    
    context.Book.Add(book);
    context.Member.Add(member);
    
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "John Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, Book = book, Member = member});
    context.SaveChanges();
}

C#
// CRUD Operations using EF Core

using Microsoft.EntityFrameworkCore;
using Week5.Context;
using Week5.Models;

using (var context = new LibraryContext())
{
    // Add new Book
    var book = new Books { Author = "John Doe", Isbn = 4568709, Title = "Harry Potter" };
    
    // Add new member
    var member = new Member { FirstName = "LeBron", LastName = "James" };
    
    context.Book.Add(book);
    context.Member.Add(member);
    
    
    // Querying db
    Console.WriteLine(new string('-',20));
    var query = await context.Book.OrderBy(b => b.BookId).FirstAsync();
    Console.WriteLine(query);
    
    //Updating db
    query.Author = "John Smith";
    query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, Book = book, Member = member});
    context.SaveChanges();
}

This works well, it was save in the db. Now, what I tried to do next, is to change Book = book from Book = book.BookId and Member = member from Member = member.MemberId.

query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, Book = book.BookId, Member = member.MemberId});

query.Lendings.Add(new Lending{DueDate = new DateTime(2025,08,08), IssueDate = DateTime.Now, Book = book.BookId, Member = member.MemberId});

But it seems I get a compiler error where

Cannot convert source type 'int' to target type

Cannot convert source type 'int' to target type

.

Ok, so, the thing, here Book and Member represent book and member objects respectively, that's why I can directly assign an int like I was doing earlier when I was explicitly setting the foreign keys.

So, question:
Here, we set the navigation properties, we don't use the foreign keys them, we could also have said BookId = book.BookId, no? would there be a difference?

F

FakerOP•2/26/25, 5:06 PM

Unhandled exception. Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Microsoft.Data.SqlClient.SqlException (0x80131904): The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Lending_Member_MemberId". The conflict occurred in database "TestDB", table "dbo.Member", column 'MemberId'.

Unhandled exception. Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Microsoft.Data.SqlClient.SqlException (0x80131904): The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Lending_Member_MemberId". The conflict occurred in database "TestDB", table "dbo.Member", column 'MemberId'.

Unhandled exception. Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Microsoft.Data.SqlClient.SqlException (0x80131904): The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Lending_Member_MemberId". The conflict occurred in database "TestDB", table "dbo.Member", column 'MemberId'.

Unhandled exception. Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Microsoft.Data.SqlClient.SqlException (0x80131904): The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Lending_Member_MemberId". The conflict occurred in database "TestDB", table "dbo.Member", column 'MemberId'.

✅ Do we need to explicitly set foreign keys in EF Core?

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